The problem is literature tends to be written by uncommon people. For an idea of colloquial (and more than that) we have the Symposia of Plato and Xenophon where we can get a good idea of banter and normal conversation between Greeks (though of the better class, it must be said). In Aristophanes we get a far more earthy colloquial language - and in verse too! But in all these cases the colloquial language is seen through literature - I doubt that many Athenians had the vocabulary (or the ability to invent it) that Aristophanes did.

you can read most of the vulgar Greek we have in ancient (not-documentary) papyri! In the unofficial papyri you can read how the Greeks spoke in their every day life. I remember a papyrus about a barkeerper who sent his daughter for beer. But instead of giving here beer, they kept the daughter, cause she was very beautiful. So the pub - holder wrote a letter to tell he was not so pleased with this and if they would not give his daughter back, that there would be a little fight, ...
Great anecdote. But also: you can imagine that a barkeeper doesn't speak like a PLato or an Aristotle. It's true that the dialogues of Plato imitate a sort of every-day-Greek, but even then it's Plato who talks in a purified vulgar Greek and this Greek also tends to litterary Greek. Papyri are the best for reading real vulgar Greek!

I don't know where to find them on the net exactly. But there is a papyrological centre in the university of Louvain (Belgium): cf. http://perswww.kuleuven.ac.be/~u0013314/cmplinks.htm for mythographic papyri. Here you have also links to many other sites about papyri. You can aslo buy texteditions in books. There are many editions, but there isn't a big corpus of texts (like the for the Greek authors) where you can find all the texts! You can search for the Corpora Papyrorum Graecarum (not finished yet!).
You have also the Corpus dei papiri filosofici greci e latini.
And there are also collections:
-Zenon papyri (here you can find the papyrus of my anecdote, etc.)
- The Oxyrhynchus papyri ...
To find your way in these labirinth of papyri there is :
Oates - Bagnall - Willis - Worp, Checklist of Editions of Greek and Latin papyri, Ostraca and Tablets, 1992

You can browse the internet for papyri with this site http://fuzzy.arts.kuleuven.ac.be/users/jan.raeymaekers/.
When you look on your left you will see: [Bronnen] and then you will also see [Papyrologische bronnen]. When you click on [Papyrologische bronnen] many links will appear for papyri on the net.

Mostly eaxh papyrus has only one edition. So I can't say that there is a good edition or a bad.
But I would start with unofficial papyri. Otherwise you does'nt have vulgar Greek neither!
The texts that I mentioned are all unofficial. So I thibnk you have material enough to begin with,

The catalogs of Papyri from Oxyrhynchus are pretty interesting. Quite a few are transcribed on Perseus so there is good cross linking. The transcriptions have lots of notes to help with the spelling changes. The POxy interface is a bit clunky: